1. Field of the Invention
The present invention relates to an object-detecting system for an optical instrument, particularly suitable for detecting a plurality of objects when automatically focusing in a camera.
2. Discussion of the Related Art
In recent years, passive systems for detecting objects have been gaining more interest than active systems as automatic focusing systems for optical instruments such as cameras. Passive systems use natural light or light reflected off an illuminated object while active systems use infrared light. Passive systems are more accurate and consume less electric power than the active systems. Passive systems can be grouped into two broad categories: trigonometrical surveying systems where the distance to the subject is detected on the basis of the external light not coming in through the taking lens, and TTL (Through The Lens) systems where the deviation of the camera from its focused condition is detected on the basis of the light coming in through the taking lens.
Both types of systems have a pair of image sensors incorporated in the optical instrument. Each image sensor receives a group of image data through a different optical path, so that two groups of image data are obtained to detect the aforementioned distance or the deviation from the focused condition on the basis of the positions of the images given by the two groups of image data relative to each other. Although the trigonometrical survey system using an external light is known to one skilled in the art, the principle is described briefly below with reference to FIG. 11.
In the figure, a pair of small lenses 11 and 12 are spaced apart by a base length b and receive the light from an object 0 through two different optical paths L1 and L2 to produce images of the object 0 on a pair of image sensors 13 and 14 at positions P1 and P2. For simplicity, it is assumed that the object 0 is located directly in front of the lens 12 when aimed through a finder. The image position P2 on the image sensor 14 is directly on the optical axis of the lens 12 while the image position P1 on the image sensor 13 deviates from the axis of the lens 13 by s as shown, unless the object 0 is at an infinite distance.
A triangle having a distance x to the object 0 for one side and the base length b perpendicular to the distance x for another side, is analogous to a triangle having a focal length f for one side and a deviation s perpendicular to the focal length f for another side. Thus, there is a relationship x/b=f/s between these two triangles. Of these variables, b and f take constant values and, therefore, the detection of the deviation s enables calculation of the distance x by x=bf/s.
The object 0 is not a point source but is a pattern source having an area. Thus, when detecting the deviation s, as many image data points as there are sensors in the right image sensor 14 are collected as a right group r of image data, and the same number of image data points are collected as a left group 1 of image data from the left image sensor 13. The image data points are collected for different positions, i.e., different values of s. The respective image data in the right group r are compared with the respective image data in the left group 1, one after another. A deviation or a shift s of the image position P1 from the optical axis of the lens 11 is determined on the basis of positions of image data in group 1 where the image data in the left group 1 coincides with that in the right group r.
In practice, the data in the left and right groups 1 and r often do not coincide with each other. Thus, evaluations of the correlation between the two groups are made for all image data groups 1 that can be sampled from the left image sensor 13, so that the shift s is determined on the basis of a sampled position of an image data group 1 indicative of a maximum correlation of all those evaluated. Additionally, it is customary that the position of the taking lens is controlled directly by the shift s rather than calculating the distance x on the basis of the shift s.
In order to detect the distance to the object and the associated deviation of focusing of an optical instrument, it is necessary to detect the object by means of a pair of image sensors. For example in FIG. 11, the image sensor 14 has a field of view with a field angle v with respect to the optical axis of lens 12. When the object 0 is within field v, it can be detected with higher accuracies as increasing amounts of data indicative of the pattern of the object are collected. Thus, the larger the field angle, the more accurately the object is detected. However, practical problems arise in connection with increasing the field of view.
Referring to FIG. 12, when the field of view v1 is large, a plurality of objects, for example 01 and 02, often fall in the field angle, and which object is actually being detected is indefinite, thereby causing an erroneous detection. Conversely, if the field angle is small as depicted by v2, there may be no object in the field of view, thereby causing failure in detecting an object.
Therefore, the field of view should be selected carefully. Conventionally, narrow field angles have often been selected to avoid erroneous detection of an object because if detection failure occurs, the user is informed that the object is not detected, so that the user may again try to detect the object. However, the user loses a good chance of tripping a shutter. The drawback associated with a narrow field angle can be solved by combining the system with a so-called focus lock function. In such a case, the operation becomes more complex and includes two stages. If the optical instrument is moved during the two-stage operation, even though it is a small movement, so-called "hand-shake" may spoil the performance of the instrument.
With the field angle held narrow, an object not on the optical axis of the optical instrument can be detected by using the distance detection technique (Published Unexamined Japanese Patent Application Nos. 60-15506 and 61-120001) in which the distance to the object is measured in an oblique direction. This technique is still not sufficient in that when a plurality of objects are involved, i.e., sufficient data showing which object should be selected may not be obtained.